1. Field of the Invention
The present invention relates to an electronic system for detecting, tracking and maintaining a particular frequency. Specifically, the invention is used to detect and track acoustic resonance frequencies for use in conjunction with a photoacoustic spectroscopy apparatus.
2. Description of the Prior Art
It is well known that a tunable laser can output a wide variety of wavelengths and that many organic and inorganic compounds possess strong absorption bands which can uniquely identify the unknown compound.
In an acoustically resonant photoacoustic spectroscopy apparatus, a tunable laser beam is directed into a photoacoustic resonant cell which contains a gas sample. The wavelength of the laser beam is chosen to be coincident with an infrared absorption feature of the species of gas to be detected.
The gas molecules of interest are vibrationally excited by the laser beam, and the energy absorbed by these molecules is periodically transferred to nonabsorbing surrounding gas molecules. This produces a modulated pressure rise and a resulting pressure wave which can be detected by a pressure transducer. If the laser amplitude is modulated at frequencies in the audio region, a sensitive microphone in a photoacoustic cell can be used as the pressure transducer.
However, if there are only trace amounts of gas to be detected, the resulting pressure wave will be quite small and thus difficult for the cell to detect. In such a case, detection is only possible if the cell is operated at its resonant frequency. In order to have the cell operate at its resonant frequency, the impinging pressure wave must be adjusted to compensate for varying gas composition, temperature, humidity and a variety of other conditions. Measurements of the pressure wave are made by detecting and tracking the output of the cell. To modulate the laser in this way, frequency signals from the acoustic cell must be detected and tracked. Once frequency signals from the cell are detected, the cell's frequency is employed to modulate the the amplitude of the laser beam at the cell's resonant frequency.
The signal measured by the photoacoustic detector is directly proportional to the laser power absorbed by the sample. However, these signals are very small where the unknown to be detected is present in the part per billion (ppb) range. To permit detection at the ppb level, the signal must be maximized and must be detected against a null background. That signal is maximized if the cell operates at the peak of its resonant frequency curve. That can be accomplished by modulating the amplitude of the laser beam which is incident on the cell.
The prior art discloses that the functions of acoustic frequency detection, tracking and data acquisition must be Performed manually by skilled personnel, who are required to utilize expensive laboratory equipment. A phase sensitive detector and a variable frequency optical chopper have been employed in such measurements. The operational procedures consisted of searching for the unknown frequency of the photoacoustic resonant cell while simultaneously adjusting the frequency of the phase sensitive detector and the optical chopper.
These techniques are expensive and time-consuming and suited only for the laboratory. This is especially true because the prior art techniques are not suited to use in field environments where the operator must make manual adjustments to compensate for varying gas composition, temperature, humidity and other environmental conditions.
It is therefore an object of the present invention to automatically detect and track acoustic cell frequency.